154 ARMILLARIA MELLEA, 
a thick white flaky mass of mycelium which usually spreads 
in the region occupied by the cambium. Compared with 
the very thin layers of mycelium produced by Fomes annosus 
the mycelium of Armillaria is almost leathery in consistency. 
Like the rhizomorphs it is composed of a central loose 
medulla and a denser cortex. The cortex is composed of 
hyphae running longitudinally and cut up into almost 
isodiametric segments by very numerous septa. Branches 
are given off by these hyphae, and these branch hyphae 
grow out into the surrounding tissues, attacking the phloem 
on one side and the wood on the other, chiefly by way of 
the medullary rays. It has been stated that the felty 
mycelial layer generally grows in the region of the cambium ; 
sometimes, however, it is outside the phloem in the inner 
cortex, or among the outer phloem cells. In either case, 
further felty mycelial layers are subsequently made, so that 
in pulling off the cortex and phloem of a root or stem in an 
advanced state of attack, several layers of white mycelium 
are found, one inside another. These layers are veined, and 
at the upper limit present an irregular outline. 
The cambium and phloem are usually attacked in the 
larch to a height of 3 to 5 ft. In rough-barked pines, such 
as Scots and Corsican, mycelium is often found at a higher 
level, whilst in Weymouth pine it is usually confined to the 
lowermost 2 ft. On the death of the tree the mycelium 
continues to grow as long as there is sufficient moisture. 
When the bark contracts away from the wood, leaving an 
air cavity, the mycelium takes the form of Rhizomorpha 
subcorticalis, the structure of which has already been 
described. 
In the wood the hyphae spread at first chiefly through 
the medullary rays, and they often grow sufficiently fast to 
reach to the centre of the tree from one side whilst the tree 
is still living on the other. From the medullary rays the 
hyphae grow into the tracheides and spread from tracheide 
to tracheide through fine bore-holes made in the tracheide 
walls. The hyphae naturally grow along the tracheides 
much more rapidly than across them, so that the rot spreads 
